In mobile wireless communications networks, a quality of transmissions is often impaired by deep fading, which varies in time, frequency, and/or space domain. To solve deep fading in radio communications, diversity transmission and diversity reception have been used to reduce a probability of bit errors by using multiple copies of transmitting data. The diversity transmission usually requires redundant data over different time, frequency, polarization, space, delay, or any combination thereof. For example, a delay diversity transmission uses multiple transmitting antennas and sends the same data from different antennas with delays instead of multiplexing different data at a cost of antenna redundancy and multiplexing loss. The multiple copies of the same data provide a diversity gain, which reduces the probability of bit errors occurred at a time when the deep fading significantly reduces the channel quality.
For frequency-selective fading, some radio networks have adopted orthogonal frequency-division multiplexing (OFDM) based on a discrete Fourier transform (DFT), which converts a frequency-selective channel into multiple frequency-flat sub-channels in the Fourier basis. Although the OFDM can simplify a receiver by avoiding a need of channel equalization for inter-symbol interference, the diversity gain cannot be obtained without an additional coding across the frequency domain. Because such a coding over the frequency domain can decrease a data rate due to parity bits overhead, a full-rate transmission and diversity gain cannot be achieved at the same time. In order to achieve higher diversity gain, wideband signal transmissions have been used so that the signal bandwidth is wider than a coherence bandwidth of the frequency-selective fading channels to obtain independently faded multiple data.
In recent years, machine-to-machine (M2M) networks are increasingly used to connect billions of devices for various applications, including factory automation, intelligent transport systems, healthcare systems, and smart home appliances. For such M2M networks, reliable communications and low-latency communications are of great importance. Hence, such applications cannot use a long forward error correction (FEC) code, such as capacity-approaching low-density parity-check (LDPC) codes. As a consequence, some techniques for short-message data transmission achieving a full diversity gain have been demanded by fully exploiting time, frequency, space, and polarization resources.
Some diversity transmission methods, based on space-time coding (STC), can achieve a full rate and full diversity. However, using multiple antennas can limit the application for the cases with limited energy or computational resources. One method, described in U.S. Pat. No. 8,290,074, uses a pseudo-random phase precoding (PRPP) to reduce the complexity of adaptive precoding and to achieve the full diversity gain. However, such random precoding methods have no guarantee to keep the transmission signal power normalized, and to maintain the codeword distance large enough.
In a similar context, unitary random phase precoding is used by a method described in U.S. 2014/0247894 to solve the power fluctuation problem. However, such a precoding is also random and requires a feedback link so that the precoder is selected adaptively depending on channel state information (CSI) to maximize a signal power at the receiver. Such an adaptive precoding cannot work without a reliable feedback channel, especially for mobile communications networks wherein the transmitter or the receiver is moving fast.
Accordingly, there is a need in the art for a method that is suitable for transmission over fading channels, and simultaneously improves diversity gain while maintaining the data rate.